A number of processes in the liquid, gaseous or supercritical phase have been described in science and industry for the conversion of alcohols by reaction with ammonia into primary amines.
The challenge for these processes is to achieve high selectivities to the primary amine. Since alkylamines are more nucleophilic than ammonia and their nucleophilicity increases with the number of alkyl groups on the nitrogen atoms, secondary and tertiary linear amines are preferentially formed. In addition, a limited selectivity to the diamine has been observed when diols are used as starting material in such reactions because appreciable amounts of intermediate amino alcohol have frequently also been isolated. In the case of relatively short-chain diols, cyclization as alternative reaction also plays a significant role (e.g. Fischer et al., Catal. Today 1997, 37, 167-189).
The reaction in the gas phase is possible for readily vaporizable lower alcohols and diols. It is carried out over heterogeneous catalysts in the presence of NH3 and H2. The high temperatures of up to 400° C. and pressures of up to 300 bar which are necessary frequently bring about the formation of undesirable intermediates, by-products and subsequent products, e.g. secondary and tertiary amines, alkenes and alkanes (by means of dehydration/hydrogenation), cyclic species and in the case of diols also amino alcohols. Direct aminations over heterogeneous catalysts have also been carried out in the liquid phase; in some cases, it is difficult to distinguish between gas and liquid phase on the basis of the available data. Examples of the reaction over heterogeneous catalysts are the patents and literature references mentioned below.
Thus, EP 0963975 describes the direct amination of, inter alia, primary alcohols and 1,2- to 1,6-diols over oxidic ZrO2-supported Cu—Ni—Co catalysts in the presence of hydrogen. Depending on the reaction conditions, amino alcohols, cyclic compounds or diamines can be obtained, with the diamine selectivities described being low. In DE 1543377, C4-C8-diols are hydrogenatively aminated to the diamines over Co—Cr—Mn catalysts in the presence of P4O10, sometimes under hydrogen pressures of up to 300 bar. In this way, 86.5% of hexamethylenediamine can be prepared from 1,6-hexanediol in a single pass. DE 102006061045 (over Ni—Cu/ZrO2 catalysts) and DE 102006061042 (over Ni—Cu—Ru/ZrO2 catalysts) describe the hydrogenative amination of alcohols or dihydroxy and polyhydroxy compounds in the range from 180° C. to 220-250° C., but preferably of stearyl alcohols.
WO 9638226 describes the direct hydrogenative amination of, inter alia, C2-C6-alcohols and -diols, also those having further functional groups, by means of ammonia over Re—Cu—Ni—Co catalysts and/or Ru catalysts. WO 2007093514 and WO 2007093552 describe the hydrogenative amination of ethylene glycol over Ru—Co catalysts; ethylenediamine was isolated in yields of up to 57% together with further products. In the examples, only monoethanolamine is used as substrate. Cyclohexanol is reacted with NH3 and 200 bar of H2 at 260-300° C. over Ca aluminosilicates to form cyclohexylamine. The preparation of various primary monoamines is described in DE 19859776 (over Cu—CuO/TiO2 catalysts), WO 2008072428 (over Ru/ZrO2 catalysts) and WO 2007077903 (over Ru/Al2O3 catalysts) at reaction temperatures of 180-250° C.
Various diether or polyether diols have likewise been subjected to direct hydrogenative aminations using ammonia. DE 3903367 describes the amination of diethylene glycol over oxidic Zr—Cu—Ni—Co catalysts, giving aminoethoxyethanol and morpholine as main products. Polyether diols are hydrogenatively aminated directly over Raney Ni or Raney Co catalysts at 220-250° C., with 0.06-0.12% of higher amines being formed. The preparation of polyetheramines in U.S. Pat. No. 4,153,581 is carried out at 140° C. over Co—Cu catalysts which contain Zn, Zr or Fe as further active components; here, the proportion of aminated products in the reaction mixture is only 12-60 percent by weight.
Baiker and coworkers have published a study on the continuous direct amination of propanediol in supercritical ammonia (Angew. Chem. Int. Ed. 1999, 38, 351-354). Here too, the problems of the selectivity to the diamine, which does not exceed 20%, are discussed.
The additional use of hydrogen in a hydrogenative amination in which the use of heterogeneous catalysts is necessary is costly. Such processes are not suitable for relatively long-chain linear aliphatic alcohols and diols because the at least partial decomposition of starting material and product under the reaction conditions required would make the economics questionable. In addition, the achievable selectivities to the diamine are not competitive for applications in the polyamide field.
Only very few examples of the homogeneously catalysed direct amination of primary and secondary alcohols in liquid phase are conventionally known. Here, the use of ruthenium catalysts permits the concept of “borrowing hydrogen” (Williams et al. Adv. Synth. Catal. 2007, 349, 1555-1575), i.e. hydrogen does not have to be additionally introduced since the H2 equivalent which is initially liberated during the dehydrogenation of the alcohol in the first reaction step is “parked” on the catalyst and is reintroduced into the cycle in a later phase. Milstein and coworkers (Angew. Chem. Int. Ed. 2008, 47, 8661-8664) report the selective reaction of monohydric, including functionalized, alcohols in the liquid phase in the presence of excess ammonia and the ruthenium-PNP pincer complex carbonylchloro[4,5-bis-(diisopropylphosphinomethyl)acridine]hydridoruthenium(II). The yields of primary amine were 78-96%. The reactions were carried out in a solvent at 7.5 bar, 135-180° C. over a period of 12-36 hours. Polyhydric alcohols were not aminated, nor were secondary alcohols. In addition, WO 2010018570 also describes the use of quinolinyl-based PNP pincer ligands, with comparable results.
Beller and coworkers (Angew. Chem. 2010, 122, 8303) and Vogt and coworkers (Angew. Chem. 2010, 122, 8307) both describe the preparation of primary amines from secondary alcohols in yields of up to 93% using a catalyst prepared in situ from the Ru and P(III) precursor compounds dodecacarbonyltriruthenium(0) and 2-(dicyclohexylphosphino)-1-phenyl-1H-pyrrole (cataCXium®PCy). Here too, no diols were used as starting materials.
It cannot be assumed from the abovementioned patent documents and literature references that the catalytic processes described can be applied in the same way to polyhydric alcohols. The abovementioned secondary reactions can play a selectivity-limiting role; in addition, oligomerizations via amino alcohols formed as intermediates are possible in the liquid phase.